Many techniques have been devised for monitoring the moisture content of a material, but most of these techniques utilize discrete samples, or batches, of material and are not applicable to a continuously flowing, or on-line, production environment. For example, known techniques for use in batch processing of materials have been described in various patents, including U.S. Pat. Nos. 3,781,673 issued on Dec. 25, 1973 to Resh, 4,050,016 issued Sept. 20, 1977 to Marsh et al., 4,147,976 issued on Apr. 3, 1979 to Wang, 4,174,498 issued on Nov. 13, 1979 to Preikschat, 4,352,059 issued on Sept. 28, 1982 to Suh et al., 4,399,404 issued on Aug. 16, 1983 to Resh, 3,979,581 issued on Sept. 7, 1976 to Reuland, and 3,778,707 issued on Dec. 11, 1973 to Vogel. None of the techniques described therein is applicable to on-line production applications wherein the material whose moisture content is to be measured is continually flowing.
In polymer processing, for example, it is generally desirable to determine moisture content with a resolution of the order of 100 parts per million (ppm). Such precise determination of moisture content in flowing materials is a particularly difficult problem in view of the multitude of factors that can corrupt the measurement. The two most important influences are those of material temperature and material packing density variation.
In those systems described in certain of the above patents which discuss the problem of changes in the packing density of the sample being measured, the proposed solutions to this problem are generally not applicable to continually-flowing, on-line systems. For example, certain patents propose solving this problem either by measuring the weight of discrete samples removed from the batch of material or by making a sample container that is configured and operated to encourage a particular packing density for each sample. Such approaches are clearly not feasible or economical for on-line moisture monitoring systems.
In addition, only the Suh et al., Preikschat, Reuland, and Vogel patents disclose the use of measurements of dielectric loss of the samples involved for monitoring moisture content. In Preikschat, for example, measurements of both dielectric loss and capacitance are made, but neither quantity is used in a way to compensate for packing density variations. It is believed that the basic reason that none of the aforesaid known systems is able to provide a satisfactory technique for compensating for packing density changes in an on-line system is that measuring the small amounts of dielectric loss present in most materials is considerably more complicated than merely measuring the dielectric constant of the material. While in systems dealing with most non-plastic materials whose moisture contents are much higher than those of plastic materials, simply measuring the effect of moisture on the dielectric constant may be sufficient. However, in systems dealing with plastic materials, measurement of the dielectric constant alone is clearly inadequate. Moreover, the computational complexity involved in implementing a suitable technique for compensating for packing density makes it difficult to achieve such compensation in a purely analog system as such previous systems are typically configured.
While the method disclosed by Reuland incorporates dielectric loss data to compute moisture content independently of packing density variations in an on-line situation, the approximations that necessarily attend the use of polynomials of relatively low order may result in large errors when significant packing density variations occur. In principle, higher-order polynomials could be employed to extend the range over which the compensation for density variation is effective. Unfortunately, the number of calibration experiments that must be performed increases, since "n" independent experimental points are required to specify uniquely an nth-order polynomial. This empirical burden can become quite severe (i.e., "n" must be rather large) when seeking measurement resolutions on the order of 100 ppm as desired, limiting the practicality of such an approach.
Although it may not always be practical to compute moisture content accurately at all times, it is often possible to identify when errors are likely to be unacceptably large, and to provide some indication to the user as to this fact. For example, if the sensor is not filled with a sufficient quantity of material, its capacitance will be less than if the packing density were higher. Below a certain critical packing density, the relationship between loss and capacitance (viz., FIG. 3) may no longer be substantially linear, as assumed. Accordingly, it is desirable to indicate when the packing density is insufficient for accurate determination of moisture contents. As can be seen from examination of FIG. 3, a low packing density can be readily detected as insufficient capacitance, e.g., the operating regime to the left of dashed line 29. The precise location of dashed line 29 is best determined by experiment. Once determined, the moisture algorithm (described later) can be modified to signal the condition of insufficient packing density based on the measurement of capacitance.
Another critical factor in monitoring moisture contents of flowing materials concerns the temperature of the flowing material. The critical nature of the temperature measurement problem in this context is not generally appreciated by those in the art, and little attention has been paid to this problem in prior art systems. Although Reuland discusses an extension of his method that incorporates temperature information, several factors mitigate against doing so in a straightforward manner. First, a spatial temperature average is needed to account for spatial thermal gradients that inevitably arise in a flowing material. Such averaging is necessary because a temperature measurement error of as little as one degree Celsius can result in a moisture measurement error of as much as 1000 ppm in some materials.
Another critical temperature-related consideration is that of matching the dynamic behavior of the temperature measurement apparatus to the actual dynamic thermal behavior of the material under test. This matching is necessary to insure that the temperature indicated by the thermometric apparatus does in fact reflect the actual temperature of the material under test at all times, even when the material is actively heated or cooled during processing. For example, most temperature apparatus tends to respond to the temperature of both the flowing material and the air, a situation that is generally unavoidable because such apparatus is in physical contact with both the air and the flowing material. However, the volume-averaged temperature of the flowing material tends to lag behind that of the air under dynamic conditions. Hence, the temperatures as reported by the thermometric apparatus tend to lead those of the actual flowing material, introducing errors. Thus, some compensation for this mismatch in thermal dynamic behavior must be provided if accurate determination of moisture contents is to be made under dynamically varying temperature conditions.
Yet another problem related to temperature concerns a dielectric property of materials. In certain polymers, for example, there exists a region of temperature in which the dielectric properties exhibit substantially no sensitivity to moisture content, although the material may exhibit significant sensitivity to moisture at tempratures above and below this critical region. Furthermore, typically below a certain temperature, the dielectric properties of most materials exhibit substantially no sensitivity to moisture content.
U.S. patent application Ser. No. 489,319, filed on Apr. 28, 1983 by Suh et al., has proposed an on-line moisture measurement system that incorporates a measurement of dielectric loss and that provides in the overall system techniques for compensating for variations in dielectric loss in a sensor full of material due to variations in temperature and to variations in packing density of the material as it flows through a sensor element. Such a technique requires a relatively complicated sensor device which utilizes a pair of reference cells having known moisture contents, one having a substantially zero moisture content and the other having a moisture content substantially equal to the maximum moisture content expected in the material which is being processed. The difference between the dielectric losses of the materials in the first and second reference cells is used to compensate automatically for variations in loss of the sensor due to variations in temperature. Further, the system provides a signal output proportional to packing density to compensate for changes in this parameter. The signal output is combined with the measured signal which represents the dielectric loss difference between the flowing material and the material in one of the reference cells to compensate for changes in packing density.
While the latter system is the first one known to provide for automatic temperature and packing density compensation in an on-line system, certain disadvantages occur in making and using such systems. For example, it is found that the moisture content measurement is subject to errors which arise due to the technique itself as well as to changes in the characteristics of the reference cells which are used. Moreover, when a different material is utilized, a different pair of reference cells is required and the reference cells have to be replaced continually for on-line measurements of different materials. Further, even when using the same material the characteristics of the reference cells tend to change with age and for that reason also they must be periodically replaced. Additionally, the overall system tends to be physically cumbersome to install and use and relatively expensive to make and maintain.
In view of the foregoing considerations, it is evidently necessary to combine sensitive dielectric capacitance and loss measurements with sophisticated temperature measurement to compute accurately the moisture content of continually flowing materials. Further, such measurements should use techniques which are relatively easy to install and use, which do not cause large errors during use or with age, and which can be made and maintained at reasonable cost. In addition, to assure that inaccurate readings do not result from density variations or material temperatures that lie outside of some anticipated range, some indication of such a departure from normal operating regimes must also be provided.